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1-Newbie

## induction

Help!
I am having problems with this question.

Can an object exert a force on itself? When a coil induces an emf on itself, does it exert a force on itself?
13 REPLIES 13
Sure does. And if you are not careful and put too much current through it it can explode. I don't have the URL handy but there are people out there making a hooby of squashing coins using very high magnetic fields. Their coils are typicallly single use as they explode when used.

Tom Gutman
3-Visitor
(To:TomGutman)
On 3/31/2004 1:48:05 AM, Tom_Gutman wrote:
>Sure does. And if you are not
>careful and put too much
>current through it it can
>explode. I don't have the URL
>handy but there are people out
>there making a hooby of
>squashing coins using very
>high magnetic fields. Their
>coils are typicallly single
>use as they explode when used.

Bert Hickman is perhaps the most widely known of the coin shrinkers...
http://205.243.100.155/frames/shrinkergallery.html

Tom, how would you geometrically model a force applied against its own source? Is it really that the coil is exerting a force on itself, or is it that the copper atoms are exerting a force against each other?

Dave
At a low level, and ignoring the original growth of the field, you should have the conduction electrons, the moving charges responsible both for the current flow and the magnetic field, exerting a force on any moving charges. During the growth of the field the field should be excerting a force on all charges. I'm not sure how that models in terms of the individual particles and how one balances Newton's third law for that.

Tom Gutman
7-Bedrock
(To:TomGutman)
In terms of a lumped element inductance (magnetic energy storage device), the forces generated are in the direction of reducing the energy, which for a coil, means a smaller diameter. The force tries to contract the coil (squeeze a coin?). For a lossless system, the force has a general form of dE(I,x)/dx, where E(I,x) is the stored energy as a function of current I and spatial position x. In general, the functional form of the energy depends on the "independent" variables. Electrically, the magnetic state may be expressed in terms of current I or flux Lambda, and it matters in general. Conveniently, if the system is linear, it doesn't matter.

For a circular coil, the energy will be a function of the radius r, and the derivative will need the appropriate Jacobian transformation, but the idea is the same. The force depends on the derivative of energy as a function of the spatial variable(s).

The same general electromechanical relations explain the operation of motors, generators, capacitor microphones, and myriad other things. If you have heard the machine gun tapping and banging in an MRI machine, then you have experienced the (unwanted) acoustic side effects of a gradient coil contracting when a current is applied. (The currents are pulses of a few hundred amperes in coils on the inside of the bore, with inductances on the order of 0.3-1mH.) This is a vivid example of a self-contracting directed force generated by current in a coil.

Lou
1-Newbie
(To:LouP)
Hi,

Our advaced research team search the CFD math tools for subsonic, supersonic, hypersonic simulation and modelling approach solutions as
flow energy transportation. But multiphysics from today hav't exactly solution, abstracts without of error to this problem.
1-Newbie
(To:LouP)
Hi,

Our advaced research team search the CFD math tools for subsonic, supersonic, hypersonic simulation and modelling approach solutions as
flow energy transportation. But multiphysics from today hav't exactly solution, abstracts without of error to this problem.
3-Visitor
(To:TomGutman)
Hi Tom and Lou,

The current is due to moving electrons. But the ripping apart of the copper means either the Van der Waals force is overcome at the copper's molecular level, or the strong force is over come at the copper's atomic level. Does the electron have the strength necessary to rip apart molecules or atoms?

Also, unless the strong force and Van der Waals force are magnetic in nature, I don't see how the magnetic field generated by an intense electromagnetic field could affect the nuclear structure of the copper. There has to be a mechanism for magnetic fields generated by electron current to affect atomic or molecular structure.

Dave
>Is it really that the coil is exerting a force on itself, or is it that the copper atoms are exerting a force against each other ?
>______________________

Atoms have nothing to do as you figure. They just vibrate, i.e: transmit perfect chock at 0 resistance. Atoms in AC wiring don't displace, however they exhibit a kind of potential looking like DC current would electrolyse the area in contact + galvanic effect. The balancing force is the orthogonal magnetic field.

jmG
On 3/31/2004 1:10:13 AM, klemj0034 wrote:
>Help!
>I am having problems with this
>question.
>
>Can an object exert a force on
>itself?
NO

When an emf is SELF induced in a coil,
THE LINKED MAGNETIC FLUX IS CHANGING WITH TIME AND THE EMF IS A VOLTAGE APPEARING TO OPPOSE THE INCREASE/DECREASE IN COIL CURRENT. V = L dI/dt.
emf = -LdI/dt

does it exert a force on itself? YES, BUT NOT A NET BODY FORCE. THE SUM OF MAGNETIC FORCES ON A SMALL SECTION OF CURRENT CARRYING WIRE IS GIVEN BY dF = IdL x B. HERE I IS THE CURRENT IN THE WIRE, dL IS THE LENGTH OF THE SEGMENT, IdL IS TAKEN TO HAVE VECTOR PROPERTIES WITH THE LENGTH OF THE VECTOR IdL IN THE I DIRECTION AND PARALLEL TO THE WIRE, WITH B EQUAL TO THE RESULTANT MAGNETIC FIELD DUE TO ALL OTHER WIRE SEGMENTS CARRYING THE SAME CURRENT I. THE RESULT IS THAT LINEAR, CYLINDRICAL COILS COMPRESS LENGTHWISE AND EXPAND RADIALLY IF THEY ARE ALLOWED. SUCH DEFORMATION LOWERS THE ENERGY STORED IN THE MAGNETIC FIELD FOR A GIVEN CURRENT.
--------------------

---------------------
Physics: Common Sense made Obscure by Mathematics Don Sparlin
4-Participant
(To:klemj0034-disab)
On 3/31/2004 1:10:13 AM, klemj0034 wrote:
>Help!
>I am having problems with this
>question.
>
>Can an object exert a force on
>itself? When a coil induces
>an emf on itself, does it
>exert a force on itself?

Is this two questions?
1) every action has an equal and opposite reactions
2) current and voltage are independant, even in a coil.

An object, by expending energy can create an effective force, but you need to dream up the particular case. e.g. eskimo on top a perfectly smooth ice igloo, how to s/he move away from that balance point and slide down the side?

The induced voltage creates no current unless there is a circuit loop, and that circuit loop itself is another inductor coil!

Oh, and it is a loop aerial as well....

Philip Oakley
Philip,

>The induced voltage creates no current unless there is a circuit loop, and that circuit loop itself is another inductor coil!

Oh, and it is a loop aerial as well....

Philip Oakley
>_____________________________

Not exactly like that simple.

> When a coil induces
>an emf on itself, does it
>exert a force on itself ?

.... certainly yes and a lot of force if the magnetic field is collected by appropriate material like soft iron+silicon special alloy. So much force and vibrating force that the coil has to be embedded into special varnish. In big turbo machines, the coils is copper tubing cooled by CO2 or other cooling media. The life time is predictable from experience and the winding redone periodically.

Same principle applies to aerial but differently as there is some other resistance in play. Near an emitting aerial you feel like if an invisible humming bird would be there as the aerial vibrates. Birds passing to close drop dead fried. Don't try either !

Other effects are health hazard depending upon the frequency or DC. As you approach the bus bar of a magnesium plant sucking 380000 Amp, whatever is magnetic, even little, flies away: tools out of the pocket, watches done, calculators done, glasses out of the nose ... don't stay too long around. The deserving robot is encapsulated in a double Faraday cage ... etc.

Under just the right wind you might see lose wiring between poles touching, generally it results in power failure. Have you been near a big transfo they are not silent. Electric lines are humming... etc.

Magnetic induction is always present and can't be controlled. When carrying signals, the trick is to twist the two wires together, very accurately to increase the CMR (Common Mode rejection), in other words to increase the S/N (Signal to Noise ratio). It does not take much imperfection to create "ground loop current" ... a pest in Instrumentation and any hard wired system.

jmG

On 3/31/2004 1:10:13 AM, klemj0034 wrote:
>Help!
>I am having problems with this
>question.
>
>Can an object exert a force on
>itself? When a coil induces
>an emf on itself, does it
>exert a force on itself?
------------------------
Recall that there are two types of forces.
A coil with a magnetic field due to its current feels a BODY force in an external magnetic field gradient.

A coil with a magnetic field due to its current feels INTERNAL forces which seek to increase the diameter of the loop and shorten the length of a finite coil.

This goes along with the fact that massive objects feel internal gravitational forces of compression, but no acceleration without an external source of gravitational field.

Finite size charged objects feel internal Coulomb forces which, acting alone, would disperse the charge due to the repulsive forces. Again, no center of mass acceleration without an external electric field.

---------------------
Physics: Common Sense made Obscure by Mathematics Don Sparlin
On 3/31/2004 1:10:13 AM, klemj0034 wrote:
>Help!
>I am having problems with this
>question.
>
>Can an object exert a force on
>itself? When a coil induces
>an emf on itself, does it
>exert a force on itself?

There are lots of examples of objects "exerting force" on themselves. Consider the simple two-plate air capacitor that's charged to some voltage. The opposite charges on the plates are attracted to each other and the force attempts to bring the plates together, but they're held apart by the real directric in a real capacitor.

TTFN,
Eden
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